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// Copyright 2020 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
package attest
import (
"bytes"
"crypto/x509"
"errors"
"fmt"
"github.com/google/go-attestation/attest/internal"
)
// SecurebootState describes the secure boot status of a machine, as determined
// by processing its event log.
type SecurebootState struct {
Enabled bool
// PlatformKeys enumerates keys which can sign a key exchange key.
PlatformKeys []x509.Certificate
// PlatformKeys enumerates key hashes which can sign a key exchange key.
PlatformKeyHashes [][]byte
// ExchangeKeys enumerates keys which can sign a database of permitted or
// forbidden keys.
ExchangeKeys []x509.Certificate
// ExchangeKeyHashes enumerates key hashes which can sign a database or
// permitted or forbidden keys.
ExchangeKeyHashes [][]byte
// PermittedKeys enumerates keys which may sign binaries to run.
PermittedKeys []x509.Certificate
// PermittedHashes enumerates hashes which permit binaries to run.
PermittedHashes [][]byte
// ForbiddenKeys enumerates keys which must not permit a binary to run.
ForbiddenKeys []x509.Certificate
// ForbiddenKeys enumerates hashes which must not permit a binary to run.
ForbiddenHashes [][]byte
// PreSeparatorAuthority describes the use of a secure-boot key to authorize
// the execution of a binary before the separator.
PreSeparatorAuthority []x509.Certificate
// PostSeparatorAuthority describes the use of a secure-boot key to authorize
// the execution of a binary after the separator.
PostSeparatorAuthority []x509.Certificate
// DriverLoadSourceHints describes the origin of boot services drivers.
// This data is not tamper-proof and must only be used as a hint.
DriverLoadSourceHints []DriverLoadSource
// DMAProtectionDisabled is true if the platform reports during boot that
// DMA protection is supported but disabled.
//
// See: https://docs.microsoft.com/en-us/windows-hardware/design/device-experiences/oem-kernel-dma-protection
DMAProtectionDisabled bool
}
// DriverLoadSource describes the logical origin of a boot services driver.
type DriverLoadSource uint8
const (
UnknownSource DriverLoadSource = iota
PciMmioSource
)
// ParseSecurebootState parses a series of events to determine the
// configuration of secure boot on a device. An error is returned if
// the state cannot be determined, or if the event log is structured
// in such a way that it may have been tampered post-execution of
// platform firmware.
func ParseSecurebootState(events []Event) (*SecurebootState, error) {
// This algorithm verifies the following:
// - All events in PCR 7 have event types which are expected in PCR 7.
// - All events are parsable according to their event type.
// - All events have digests values corresponding to their data/event type.
// - No unverifiable events were present.
// - All variables are specified before the separator and never duplicated.
// - The SecureBoot variable has a value of 0 or 1.
// - If SecureBoot was 1 (enabled), authority events were present indicating
// keys were used to perform verification.
// - If SecureBoot was 1 (enabled), platform + exchange + database keys
// were specified.
// - No UEFI debugger was attached.
var (
out SecurebootState
seenSeparator7 bool
seenSeparator2 bool
seenAuthority bool
seenVars = map[string]bool{}
driverSources [][]internal.EFIDevicePathElement
)
for _, e := range events {
if e.Index != 7 && e.Index != 2 {
continue
}
et, err := internal.UntrustedParseEventType(uint32(e.Type))
if err != nil {
return nil, fmt.Errorf("unrecognised event type: %v", err)
}
digestVerify := e.digestEquals(e.Data)
switch e.Index {
case 7:
switch et {
case internal.Separator:
if seenSeparator7 {
return nil, fmt.Errorf("duplicate separator at event %d", e.sequence)
}
seenSeparator7 = true
if !bytes.Equal(e.Data, []byte{0, 0, 0, 0}) {
return nil, fmt.Errorf("invalid separator data at event %d: %v", e.sequence, e.Data)
}
if digestVerify != nil {
return nil, fmt.Errorf("invalid separator digest at event %d: %v", e.sequence, digestVerify)
}
case internal.EFIAction:
switch string(e.Data) {
case "UEFI Debug Mode":
return nil, errors.New("a UEFI debugger was present during boot")
case "DMA Protection Disabled":
if digestVerify != nil {
return nil, fmt.Errorf("invalid digest for EFI Action 'DMA Protection Disabled' on event %d: %v", e.sequence, digestVerify)
}
out.DMAProtectionDisabled = true
default:
return nil, fmt.Errorf("event %d: unexpected EFI action event", e.sequence)
}
case internal.EFIVariableDriverConfig:
v, err := internal.ParseUEFIVariableData(bytes.NewReader(e.Data))
if err != nil {
return nil, fmt.Errorf("failed parsing EFI variable at event %d: %v", e.sequence, err)
}
if _, seenBefore := seenVars[v.VarName()]; seenBefore {
return nil, fmt.Errorf("duplicate EFI variable %q at event %d", v.VarName(), e.sequence)
}
seenVars[v.VarName()] = true
if seenSeparator7 {
return nil, fmt.Errorf("event %d: variable %q specified after separator", e.sequence, v.VarName())
}
if digestVerify != nil {
return nil, fmt.Errorf("invalid digest for variable %q on event %d: %v", v.VarName(), e.sequence, digestVerify)
}
switch v.VarName() {
case "SecureBoot":
if len(v.VariableData) != 1 {
return nil, fmt.Errorf("event %d: SecureBoot data len is %d, expected 1", e.sequence, len(v.VariableData))
}
out.Enabled = v.VariableData[0] == 1
case "PK":
if out.PlatformKeys, out.PlatformKeyHashes, err = v.SignatureData(); err != nil {
return nil, fmt.Errorf("event %d: failed parsing platform keys: %v", e.sequence, err)
}
case "KEK":
if out.ExchangeKeys, out.ExchangeKeyHashes, err = v.SignatureData(); err != nil {
return nil, fmt.Errorf("event %d: failed parsing key exchange keys: %v", e.sequence, err)
}
case "db":
if out.PermittedKeys, out.PermittedHashes, err = v.SignatureData(); err != nil {
return nil, fmt.Errorf("event %d: failed parsing signature database: %v", e.sequence, err)
}
case "dbx":
if out.ForbiddenKeys, out.ForbiddenHashes, err = v.SignatureData(); err != nil {
return nil, fmt.Errorf("event %d: failed parsing forbidden signature database: %v", e.sequence, err)
}
}
case internal.EFIVariableAuthority:
v, err := internal.ParseUEFIVariableData(bytes.NewReader(e.Data))
if err != nil {
return nil, fmt.Errorf("failed parsing UEFI variable data: %v", err)
}
a, err := internal.ParseUEFIVariableAuthority(v)
if err != nil {
// Workaround for: https://github.com/google/go-attestation/issues/157
if err == internal.ErrSigMissingGUID {
// Versions of shim which do not carry
// https://github.com/rhboot/shim/commit/8a27a4809a6a2b40fb6a4049071bf96d6ad71b50
// have an erroneous additional byte in the event, which breaks digest
// verification. If verification failed, we try removing the last byte.
if digestVerify != nil && len(e.Data) > 0 {
digestVerify = e.digestEquals(e.Data[:len(e.Data)-1])
}
} else {
return nil, fmt.Errorf("failed parsing EFI variable authority at event %d: %v", e.sequence, err)
}
}
seenAuthority = true
if digestVerify != nil {
return nil, fmt.Errorf("invalid digest for authority on event %d: %v", e.sequence, digestVerify)
}
if !seenSeparator7 {
out.PreSeparatorAuthority = append(out.PreSeparatorAuthority, a.Certs...)
} else {
out.PostSeparatorAuthority = append(out.PostSeparatorAuthority, a.Certs...)
}
default:
return nil, fmt.Errorf("unexpected event type in PCR7: %v", et)
}
case 2:
switch et {
case internal.Separator:
if seenSeparator2 {
return nil, fmt.Errorf("duplicate separator at event %d", e.sequence)
}
seenSeparator2 = true
if !bytes.Equal(e.Data, []byte{0, 0, 0, 0}) {
return nil, fmt.Errorf("invalid separator data at event %d: %v", e.sequence, e.Data)
}
if digestVerify != nil {
return nil, fmt.Errorf("invalid separator digest at event %d: %v", e.sequence, digestVerify)
}
case internal.EFIBootServicesDriver:
if !seenSeparator2 {
imgLoad, err := internal.ParseEFIImageLoad(bytes.NewReader(e.Data))
if err != nil {
return nil, fmt.Errorf("failed parsing EFI image load at boot services driver event %d: %v", e.sequence, err)
}
dp, err := imgLoad.DevicePath()
if err != nil {
return nil, fmt.Errorf("failed to parse device path for driver load event %d: %v", e.sequence, err)
}
driverSources = append(driverSources, dp)
}
}
}
}
// Compute driver source hints based on the EFI device path observed in
// EFI Boot-services driver-load events.
sourceLoop:
for _, source := range driverSources {
// We consider a driver to have originated from PCI-MMIO if any number
// of elements in the device path [1] were PCI devices, and are followed by
// an element representing a "relative offset range" read.
// In the wild, we have typically observed 4-tuple device paths for such
// devices: ACPI device -> PCI device -> PCI device -> relative offset.
//
// [1]: See section 9 of the UEFI specification v2.6 or greater.
var seenPCI bool
for _, e := range source {
// subtype 0x1 corresponds to a PCI device (See: 9.3.2.1)
if e.Type == internal.HardwareDevice && e.Subtype == 0x1 {
seenPCI = true
}
// subtype 0x8 corresponds to "relative offset range" (See: 9.3.6.8)
if seenPCI && e.Type == internal.MediaDevice && e.Subtype == 0x8 {
out.DriverLoadSourceHints = append(out.DriverLoadSourceHints, PciMmioSource)
continue sourceLoop
}
}
out.DriverLoadSourceHints = append(out.DriverLoadSourceHints, UnknownSource)
}
if !out.Enabled {
return &out, nil
}
if !seenAuthority {
return nil, errors.New("secure boot was enabled but no key was used")
}
if len(out.PlatformKeys) == 0 && len(out.PlatformKeyHashes) == 0 {
return nil, errors.New("secure boot was enabled but no platform keys were known")
}
if len(out.ExchangeKeys) == 0 && len(out.ExchangeKeyHashes) == 0 {
return nil, errors.New("secure boot was enabled but no key exchange keys were known")
}
if len(out.PermittedKeys) == 0 && len(out.PermittedHashes) == 0 {
return nil, errors.New("secure boot was enabled but no keys or hashes were permitted")
}
return &out, nil
}
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